After completing both the Solar Stone and Prismatic Shell sidequests, talk to Bosch in the basement again.

Bosch: Oh! The Solar Stone?
I'll try crossing it with this right away!

The screen fades out, there are noises as of a busy construction site, and then the scene fades back in.

Bosch: It's done.
Take this with you!

Bosch: Hu hu.
Don't think I'm done already.
Bosch is really enthusiastic this time!

The screen fades out, there are noises as of a busy construction site, and then the scene fades back in.

Bosch: It's done.
Take this with you!

The Rainbow Glasses are a better version of the Solar Glasses, and the Rainbow is Crono's best weapon. See the items page for details on both items.

Bonus rewards for clearing both quests: Rainbow Glasses, Rainbow

The Black Dream is accesible in 12,000 B.C., A.D. 600, and A.D. 1000.

The era makes no difference as far as the events that happen inside, beyond a single reference to the year, but there is a very good reason to clear it first in A.D. 1000, then come back in A.D. 600, and in 12,000 B.C. yet again...

"Black Dream" plays inside, making the hi-tech environment seem creepy, and continues playing during some (but not all) battles.

There are lots of items in here, so here's a link to the items page for details.

After a group of Laser Sites (Laser Guards), Zeal awaits their arrival just inside.

わらわ (warawa), as mentioned before, is an uncommon humble feminine first-person pronoun. I find it appropriate given her dalliance with Lavos that the associated kanji, 妾, also means "concubine".

Is that the "royal we" or just poor translation there?

Zeal gestures, one of those little portals Dalton liked using opens, and a big mutant something comes out. "Boss Battle 1" starts playing, so if there was any doubt about why she summoned it...

Zeal vanishes, and the battle commences.

The Mega Mutant actually has two parts, a torso (monster ID 184) and some kind of mutated "foot" (monster ID 185). The creature likes to use status attacks, often against the enitre party, so equip protection. Other than that, it's fairly unremarkable, and both parts have standard defenses. Ayla can steal an Elixir from the torso and a Safeguard Hat from the base.

With the boss down, the party can proceed. In a few rooms, the Walls (Panels) themselves attack, but they're more than a mere nuisance because Ayla can steal Speed Capsules from them. Unfortunately, the Walls, once beaten, stay dead, so can't just be farmed until everyone's speed is maxed out.

Naturally, a variety of other monsters also appear: Glorious Shield (Incognito / PeepingDoom), extreme defenses with shield up but a pushover with it down, steal for a Muscle Ring only when the shield is up; Black Site (Martello), standard defenses, steal for a Super Ether; Lardy (Goon), absorbs Aether and Dark, steal for a Nova Armor; Ruin Golem (Synchrite), standard defenses, steal for a Gold Earring; Wandering One (Boss Orb), seals skills and items; Drifting One (Side Kick), seals skills and items, high evasion but worth 100 tech points; Metal Myu (Metal Mute), standard defenses, steal for a Super Ether; Mini Ahriman (Flyclops), strongly resists all elements, steal for a Gold Pierce; Great Goro (Cybot), standard defenses, steal for a Deluxe Bento; Twin Nom (Ruminator), nearly immune to Aether and Dark, tend to kill each other, steal for a Last Elixir; Fat Beast (Tubster), absorbs Dark, slightly weak against Fire, rarely attacks except as a counter, steal for a Power Capsule; Gazoo (Alien), standard defenses, counters normal attacks with instant death, steal for a Magic Capsule; and Nohey (Blob), immune to Dark, weak against Water, counters magic with MP Buster, steal for a Magic Ring. The selection of monsters varies by section.

In an unusual twist, each of the battles in the room after the first elevator will only occur if the party is running when passing the corresponding (invisible) trigger location. Simply walking avoids all encounters in this room, though I have no idea how you're meant to guess that. The following room has one similar battle, but most there are instead triggered by stepping on certain floor tiles that vaguely resemble land mines. While they may not be immediately obvious as hazards, at least there's some hint of what to do to avoid combat.

A chest in the room with the floor plates contains a Magic Seal, and the next room later on has a save point, a Safeguard Hat, a Nova Armor, a Haste Helm, and a few Last Elixirs. Of the two Nu here, one sells powerful and expensive healing items, and the other...

Lackey: Sheesh! How long have we
got to listen to what he says, ribbit?

Two frogs push in what look like the seats from the Sylbird.

The seats blast off and the party lands back outside.

In the next room, there's a swirling something that summons a Fat Beast. Ayla can steal a Power Capsule from it, and it drops Elixirs. Since going back to the previous room makes it come back, this is an unlimited source of both, given enough patience, and the Elixirs should take care of healing.

There's a Stardust Cape in a chest not much later, a Power Seal and Speed Capsule a few rooms after that, and another Speed Capsule in the next room. A few rooms later, monsters appear out of nowhere. Ayla can steal Magic Capsules from Gazoos, the reptilian ones with mouth tentacles, but they're gone permanently once killed. The same room has an Elixir, a Last Elixir, and another Speed Capsule in chests.

After the next save point, the floor goes funny and the Giga Mutant shows up to do battle.

Both parts are virtually immune to physical attacks, and the torso counterattacks by draining the MP you need to hurt it. Other than that, it's unremarkable. Ayla can steal a Barrier Ring from the torso (monster ID 53) and a Hit Ring from the base (monster ID 54).

A later room has more attacking Walls for Ayla to pillage, and another Speed Capsule in a chest. On the far side of the room, the Tera Mutant appears.

Like the Giga Mutant, it's virtually immune to physical attacks, but to make things worse, the base (monster ID 56) has extremely high magic defense, absorbs all elements, and counters physical attacks with a strike that lowers HP to 1! Don't worry, just keep attacking the torso (monster ID 55) with magic. It will literally suck the life out of the base. You can probably inflict damage fast enough to kill the torso off anyway, and if not, it will eventually suck the base to death. If either part dies, the other part dies automatically soon after. Ayla can steal a Muscle Ring from the torso and a Power Seal from the base.

The chests nearby contain a Last Elixir and the White Stone. Just after the next corridor, Mini Lavos R (Lavos Spawn) pops out. This one is a bit stronger than the similar creatures on Death Mountain, but the same strategy applies.

Ayla can steal a Haste Helm from the head (monster ID 110) and a Protect Helm from the shell (monster ID 111), which WILL counterattack painfully, but it's probably worth it.

The next room has five Walls for Ayla to loot Speed Capsules from. The room after the Walls is where things get interesting. Lining the pathway are what appear to be the six playable characters (other than the Magus) in stasis. At the end of the corridor, Zeal appears and "Zeal Palace" starts to play.

Queen: Ku ku ku...
What sleeps there is your future...
The dreams that might have
come true after this...
The joys and sorrows you might
have been able to earn...
Your very tomorrow!

Queen: This Black Dream flows
across all temporal dimensions...
Awaiting that time when the
great Lavos will awaken...
Your future is certain to reach
here eventually.
You have no future!
Not unless you defeat me
and stop this Black Dream!

The party surrounds her, preparing to do battle.

Queen: Come, children of man!
I shall lure you into the midst of
the great Lavos's slumber...
To an eternal dream of black...!

Time to fight. "Zeal Palace" continues playing during the battle, then fades out afterwards.

Zeal attacks nearly every round with ハレーション (Halation), which drops each character's HP to 1, but she'll hardly ever actually finish them off. Ayla can steal a Last Elixir from her up to twice. Upon losing, Zeal drops yet another Last Elixir.

Queen: I can't use my strength here...
I've come up with a good idea.
I'll stick you vermin in the Nether Relic.
I hope you're thankful...
That you can become part of this ship!
That you can become part of me!!
That you can become part of the great Lavos!!!

She magics them into a distorted dimension, and the boss music starts up. The Nether Relic (Mammon Machine) begins in an active phase...

...during which it raises defense power when attacked physically except by the Grandleon...

...raises attack power when attacked magically...

...and heals Frog (in proportion to its own attack power) when he hits it with the Grandleon (either version)...

At the end of this phase, it unleashes an attack on the entire party, which starts out weak but can be deadly if the attack power rose too much.

Then it stops messing with energy for a while, reverting to its original attack power and defense power at some point during this phase.

Eventually, it returns to the active state and repeats.

For best results, attack only with the Grandleon when it's active, and let loose with all you've got when it isn't. Ayla can steal a Last Elixir (note that stealing attempts are regarded as magic attacks and will raise its attack power).

Once it goes down, the party appears on a surface that looks like it's on the outside of the Black Dream. Zeal warps in.

Magus: Foolish...
All existence is unable to escape
from the fate of perishing...

Magus: Zeal.
A sad woman enthralled by Lavos.
As a token of my pity...

Magus: I will bring it all to an end
by my hand!

Zeal morphs into a large mask and two huge gloves, and the battle begins. Whichever music is playing continues playing for the entire battle.

In a reversal of typical three-target bosses, it's not a good idea to focus on the secondary pieces this time. The glove on the left of the screen (monster ID 177) counters all attacks against it by dropping the offender's HP to 1, the one on the right (monster ID 178) counters by stealing all of that person's MP, and both are nearly invulnerable anyway. The mask (monster ID 176) has standard defenses, but will occasionally use Halation, so be ready to heal at any time.

Ayla can, and should, steal a Prism Dress from the glove on the left, a Prism Helm from the one on the right, and a Last Elixir from the head, but be careful since the gloves will counterattack. And bring the Magus for the music! (Or avoid him if you'd rather not Dark Bomb all battle.)

Once the mask dies, it's all over, and Zeal reappears in her normal form. "Zeal Palace" plays.

She does something, the Black Dream shoots energy downward, and there seems to be a response of some kind.

The internal storyline progression tracking variable is now 0xD6, starting the final chapter, even if you skipped saving Crono. Let's finish out the scene before starting a new page, though.

The reaction from the ground increases, the Black Dream decends and disintegrates, and Lavos breaks through from the depths.

For reference, here's a partial list of possible spoils for passing through the Black Dream: 2 Safeguard Hats (steal one from Mega Mutant), Magic Seal, Nova Armor, 2 Haste Helms (steal one from Mini Lavos R), Stardust Cape, 2 Power Seals (steal one from Tera Mutant), Barrier Ring (steal from Giga Mutant), Hit Ring (steal from Giga Mutant), Muscle Ring (steal from Tera Mutant), White Stone, Protect Helm (steal from Mini Lavos R), Prism Helm (steal from Zeal hand), Prism Dress (steal from Zeal hand), a bunch of Last Elixirs (steal some from various bosses), 4 Magic Capsules (steal from Gazoos), 17 Speed Capsules (steal from Walls), and as many Power Capsules as you have the patience to steal from Fat Beasts. Obviously, that's a lot of stuff.

Clearing the Black Dream is also required to unlock New Game+.

To make things even more interesting, the Black Dream remains present in the eras before you cleared it, so you can fight Zeal twice more and steal multiple Last Elixirs, two more Prism Dresses, and two more Prism Helms! If you got the Prism Helms from the Prismatic Shell sidequest, this gives you a total of three dresses (enough for everyone who can use one) and six helms (enough for all but the Magus, whose Helmet of Despair is better anyway once his magic defense hits maximum). However, any chests you opened in the later eras will somehow be open in the earlier ones, and any one-time battles fought are nowhere to be seen, with the exception of Zeal herself. Even Lavos is already broken open at the end.

Getting rid of that eyesore would count as historical impact except that it's just undoing a previous change and putting things (more or less) back the way they were in the first place. Even so, it needed doing.